The present invention concerns generally the field of antennas intended for the transmission and reception of an electromagnetic signal. More particularly, the present invention concerns a structure of small volume forming an antenna derived from a structure known by the name of PIFA or xe2x80x9cplanar inverted-F antennaxe2x80x9d intended to be incorporated in a portable unit of small volume, such as a timepiece. The antenna structure according to the present invention is in particular intended for transmitting and receiving high frequency electromagnetic signals allowing, for example, a radio telephone communication to be assured on a mobile telephone network. The antenna structure according to the present invention may nonetheless be applied in other systems requiring a wireless communication between a portable unit and a remote transmitter/receiver station.
The present invention also concerns a timepiece, such as a watch, adapted to transmit and receive high frequency electromagnetic signals incorporating the aforementioned antenna structure and allowing, for example, a radio telephone communication to be assured with other users on a mobile telephone network.
The continued expansion of radio frequency communication systems and in particular mobile telephone systems, leads to an increasing demand for increasingly compact and light portable communication equipment. Parallel to the technological advances which have allowed the development of electronic circuits and radio circuits of small size and the development of high performance power sources, antennas of small profile adapted to be mounted on portable communication units such as cellular telephones have already been proposed. One such structure is known by the name of planar inverted-F antenna or PIFA.
FIG. 1 illustrates an example of a PIFA structure, indicated globally by the numerical reference 1, such as typically known in the state of the art. This PIFA structure 1 includes a ground plane 2, a radiating element 6 of rectangular shape whose length is approximately equal to xcex/4, where xcex is the antenna transmission/reception wavelength, arranged substantially parallel to ground plane 2, as well as a short-circuit plate 4 connecting radiating element 6 to ground plane 2 and holding said radiating element 6 at a determined distance with respect to ground plane 2.
This antenna 1 is powered by a transmission line, such as a coaxial line 8, from the back of ground plane 2. This coaxial line 8, arranged at a determined distance from short-circuit plate 4, includes an internal conductor 8a passing through ground plane 2 to connect radiating element 6, and an external conductor 8b connected to ground plane 2 and at a distance Ad from internal conductor 8a. 
A detailed analysis of the PIFA structure illustrated in FIG. 1 can be found in the document xe2x80x9cAnalysis, Design and Measurement of Small and Low-Profile Antennasxe2x80x9d, Artech House, Norwood, Mass., 1992, Ch. 5, pages 161-180, Kazuhiro Hirasawa and Misao Haneishi, which is incorporated here by reference.
Because of its small profile, this PIFA structure is thus perfectly suited to being mounted on a small communication unit, such as a cellular telephone. This PIFA structure is thus generally mounted on a face of the conductor body of the cellular telephone which also forms the ground plane of the antenna. It will be noted that the features of antenna (resonance frequency, bandwidth. etc.) depend not only on the dimensions of the radiating element and its distance from the ground plane, but also on the dimensions of the conductor body forming the ground plane of the antenna. The shape and dimensions of this body do not constitute very restricting elements in a cellular telephone since the latter nonetheless remains an object of relatively large size with respect, for example, to a portable unit such as a timepiece which is characterised by a substantially smaller volume.
In order to incorporate such an antenna in a portable unit of small volume such as a watch, one has thus to find solutions which advantageously combine compactness and yet performance allowing radio communication to be assured in the best conditions.
One object of the present invention is thus to propose an antenna easily able to be arranged and mounted inside a portable communication unit of small volume, such as for example, a timepiece.
Another object of the present invention is to propose an antenna answering the aforementioned definition whose structure can be made as easily as possible in order to limit manufacturing costs.
The present invention therefore concerns an antenna structure intended for a portable unit of small volume, such as a timepiece, whose features are listed in claim 1.
Advantageous embodiments of the present invention are the subject of other claims.
The present invention also concerns a timepiece incorporating the aforementioned antenna structure.
One advantage of the present invention lies in the fact that the proposed structure is completely integrated in a structure of small volume which combines compactness, rigidity and aesthetics, the radiating element forming an integral part of a case forming the ground plane of the antenna. The sizing, connection and adaptability of the features of the antenna (frequency, bandwidth, impedance, etc.) are also easy.
The case forming the ground plane of the antenna also advantageously allows all or part of the electronics of the portable communication unit to be housed inside the antenna structure, this case thus forming a shielded housing inside which the electronic circuit is protected and in no way affects the performance of the antenna. The proposed antenna can thus very advantageously be powered from the inside of the housing thereby formed.
The proposed structure also combines the ground plane of the antenna and a housing allowing not only the electronic circuit of the portable unit but also any other components which may be present in said unit to be accommodated. The structure according to the present invention may thus advantageously form a watch case also allowing a timepiece movement to be accommodated.
If aesthetic criteria so demand, the structure may also be easily surrounded by or embedded in an external plastic element. This external element may thus take various forms, such as for example, the form of a middle part of a watch.
The radiating element of the antenna is small electrically, but the influence of the complete structure (including the case) determines the antenna""s radiation properties. Associated with small losses due to the absence of a dielectric between the radiating element and the ground plane, or more exactly to the presence of a dielectric formed solely by air, this leads to remarkable antenna gain and efficiency.